Rotary die cutters include a die drum or cylinder having on its surface a cutting die typically made of steel rule for cutting or creasing corrugated board against an anvil drum or cylinder. This process occurs as the board passes between the two drums. The anvil cylinder is circumferentially covered with 10 inch to 20 inch wide and initially 0.420 inch to 0.300 inch thick urethane blankets. As knives on the die drum cut the corrugated boards, the anvil urethane blankets wear down and change thickness.
It is well known that the surface speed of the anvil affects the accuracy of the die cut of the corrugated board. Ideally the surface speed of the anvil drum should be equal to the linear speed of the board as it travels through the die cutter. As the urethane blankets wear, the overall diameter of the anvil drum decreases which reduces the surface speed of the anvil and ultimately changes the cut size of the produced corrugated box.
Several systems exist to compensate for the change in diameter of the anvil by changing the rotational speed of the anvil drum in accordance with the change of the drum diameter. On some systems the die cutter operator manually measures the diameter of the anvil drum and then inputs the measurement into the control system, which then changes the rotational speed of the anvil drum. To provide for the fine anvil speed adjustment, an operator usually tries to “fool the system” and inputs a number in the control system that is higher or lower then the number corresponding to the real anvil diameter. This method is not accurate and requires several “trial and error” attempts. Also, a significant change of this number results in a large difference between the surface speeds of the die drum and the anvil drum, which leads to additional stresses on the die cutter components and the breaking of the die-cutter knives. Also, there are no provisions for the control system to “remember” this number, so when the same die is used the next time, the operator must repeat the “trial and error” procedure.
An improved system (the MicroGrind™ system) is disclosed in the above-identified patent application Ser. No. 09/471,011 where the position of the anvil grinding or trimming mechanism determines the diameter of the anvil drum automatically. The computer feeds this information into the control system, which then changes the rotational speed of the anvil drum. However, even if the surface speed of the anvil drum is perfectly correlated with the drum diameter, there is still a need to fine adjust the surface speed of the anvil drum within a small range, usually +/−3%, to achieve a perfectly sized corrugated box. There are several reasons for this need. One of them may be an imperfection of the cutting and creasing die that is mounted on the surface of the die drum. Another may be the change in the amount of impression of the die cutter knives and blades into the anvil.